The present invention is directed generally to solid materials which are capable of reversibly absorbing hydrogen gas. These materials can be utilized for a variety of applications. The applications include on board storage of hydrogen for vehicle fuel supplies, storage of hydrogen during transport to other facilities, storage of hydrogen for future use as a fuel to power machinery and equipment and storage of hydrogen for powering fuel cells for producing electricity. These materials can be utilized in heat pump systems and gas recovery systems.
In recent years considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel. It is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas weighs about thirty times the weight of a container of gasoline.
Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below -253.degree. C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Storage of hydrogen as a solid hydride can provide a greater percent weight storage than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
A high hydrogen storage capacity per unit weight of material is an important consideration in applications where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of the vehicle making the use of such materials impractical. A low desorption temperature is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
The whole field of hydrogen storage material generally has been predicated on the idea that a particular single phase crystalline host material is required. Such prior art materials, however, have not been able to meet the above noted requirements necessary for wide scale commercial acceptance. A basic limitation of many of the prior art materials has been their low hydrogen storage capacity relative to the weight of the material. Another limitation is that many of these materials possess desorption temperatures which are undesirably high for many applications particularly those where engine exhaust heat is to be used to provide the desorption energy. Many of the prior art materials also have been quite susceptible to poisoning by exposure to contaminants in the hydrogen gas or from the ambient environment. For example, many crystalline materials can be poisoned by the presence of oxygen in parts per million concentrations. Once contaminated, the storage characteristics of the materials degrade significantly rendering these materials unacceptable for use without reactivating the materials, which is expensive and complicated. Resistance to contamination or poisoning is particularly necessary when using low cost, low grade hydrogen. For example, a low cost, low grade hydrogen is formed as a by-product of the chlorine-caustic industry. Also, in the fuure, hydrogen produced from coal may become widely utilized. Hydrogen produced from coal or as a by-product of the chlorine-caustic industry can have impurities which are capable of poisoning many of the prior art hydrogen storage materials.
In order to understand the limitations of the single phase crystalline materials of the prior art, it is necessary to understand the mechanics of storing hydrogen, in particular, as they relate to hydrogen storage in single phase crystalline host materials. The concepts generally utilized in dealing with these crystalline single phase materials are that hydrogen is stored in accordance with a simple reversible reaction of the metallic material M and gaseous H.sub.2 to form a solid metal hydride MH.sub.n, where n is an integer: ##EQU1## In one proposed theory, the mechanics of hydrogen storage are when gaseous hydrogen is brought in contact with a metal that forms a hydride, some of the hydrogen molecules are dissociated into hydrogen atoms by interactions with catalytically active sites appearing on the crystalline surface. Once the hydrogen is dissociated it is then in a form which allows storage within the single phase crystalline host material. The hydrogen atoms enter the crystal lattice of the host material and occupy specific sites among the metal atoms. Such storage sites in a crystalline structure are called interstitial sites. These sites must have a certain minimum volume in order to easily accommodate the hydrogen atom. This imposes a severe limitation on the number of hydrogen atoms that can be stored.
As the pressure of the gas is increased a limited number of hydrogen atoms are soluble in the single phase crystal structure. As the pressure is further increased a metal hydride phase occurs and the material begins to absorb larger quantities of hydrogen. At this time the material is a mixture of metal M and metal hydride MH.sub.n. In theory, ultimately all of the original hydrogen-saturated metal phase will be converted into the metal-hydride phase. Very few of the single phase crystalline host materials, however, possess significant hydrogen storage capacities. None possess commercially acceptable capacity, while simultaneously possessing all the other hydrogen storage characteristics which are required for widespread commercial utilization. A major limitation of single phase crystalline materials is the result of a relatively low density of catalytically active sites and also a low density of interstitial storage sites in such materials. The low density of such sites provide materials with less than desirable kinetics and hydrogen storage capacities.
In addition to a low density of catalytically active sites, the density of hydrogen storage sites or interstitial sites is limited due to specific stoichiometries in the single phase crystalline host structures. In single phase crystalline host materials the catalytically active sites are relatively very limited in number and result from accidentally occurring surface irregularities which interrupt the periodicity of the single phase crystalline structure. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign absorbates. These irregularities typically only occur in relatively few numbers on the surface of a single phase crystalline material and not throughout its bulk.
The density of catalytically active sites can be increased to a limited extent by mechanical cracking of the single phase crystalline structure or by forming a powder therefrom to increase the surface area. Also, powders can present a utilization problem, because when the stored hydrogen is later released from the hydride, the powder particles may be included in the hydrogen gas as it is pumped to its point of utilization as a fuel.
The prior art metallic host single phase crystalline hydrogen storage materials include magnesium, magnesium nickel, vanadium, iron-titanium, lanthanum pentanickel and others. No such prior art material, however, has all of the required properties i.e., storage capacity, acceptable desorption temperatures, etc. required for a storage medium with widespread commercial utilization. For example, a crystalline magnesium hydride is theoretically capable of storing hydrogen at approximately 7.6% by weight computed using the formula: percent storage=H/H+M, where H is the weight of the hydrogen stored and M is the weight of the material to store the hydrogen (all storage percentages hereinafter referred to are computed based on this formula). While a 7.6% storage capacity is suited for on board hydrogen storage for use in powering vehicles, magnesium's other hydrogen storage characteristics make it commercially unacceptable for widespread use.
Magnesium is very difficult to activate. For example, U.S. Pat. No. 3,479,165 discloses that it is necessary to activate magnesium to eliminate surface barriers at temperatures of 400.degree. C. to 425.degree. C. and 1000 psi for several days to obtain a reasonable (90%) conversion to the hydride state. Furthermore, desorption of such hydrides typically requires heating to relatively high temperatures before hydrogen desorption begins. The aforementioned patent states that the MgH.sub.2 material must be heated to a temperature of 277.degree. C. before desorption initiates, and significantly higher temperatures and times are required to reach an acceptable operating output. The high desorption temperature makes the magnesium hydride unsuitable for many applications, in particular applications wherein it is desired to utilize waste heat for desorption such as the exhaust heat from combustion engines.
The other aforementioned single phase crystalline materials also have not achieved commercial acceptance. Irontitanium hydride has a very low hydrogen storage capacity of only 1.75%. Other single phase crystalline prior art hydrogen storage materials all provide generally unacceptable storage capacity, for example Mg.sub.2 NiH.sub.2, VH.sub.2, LaNi.sub.5 H.sub.7 have theoretical capacities of only 3.16%, 2.07% and 1.37%, respectively. In some of the prior art materials, another disadvantage has been the lack of ability to charge the materials in an acceptable amount of time. In summary, none of the prior art single phase crystalline host structures provide hydrogen storage material which has received commercial acceptance.
The whole field has been predicated on the fact that a particular crystalline structure is required, for example see Hydrogen Storage in Metal Hydride, Scientific American, Vol. 242, No. 2, pp. 118-129, February, 1980.
The present invention concerns itself with the influence of disorder on the properties of hydrogen storage materials, which premise is entirely different than that which has previously been utilized in the field.